1. Field of the Invention
This invention relates to the detection of cavitation in rotating machinery, and is particularly concerned with a system for detecting cavitation in turbomachinery and ducts, using forced-excitation ultrasonic means.
2. Description of the Prior Art
Unstable vapor pockets, known as cavities, form in rotating machinery whenever the difference in velocity head and hydrostatic pressure of the fluid therein falls below the vapor pressure of the liquid. These cavities collapse and cause erosion.
Cavitation erosion is a major problem in chemical and power-plant operations. It occurs commonly in turbomachinery including both pumps and water turbines, as well as valves, elbows and other ducting elements, and this leads to equipment damage and expensive plant unavailability. In the operations of such plants, a pump/turbine may go from noncavitating to cavitating conditions when one or more parameters such as speed, flow-rate, inlet pressure, and vapor pressure (due to weather) are changed. Detecting a severely cavitating condition prior to hardware damage and/or pump hydraulic head breakdown would greatly enhance smooth plant operation.
Most of conventional cavitation detection methods are undesirable in plant environments. Dye tests require costly disassembly and reassembly of equipment. Optical methods require windows, which can break or become soiled, creating potential hazards and establishing the need for frequent inspection, maintenance, repair or replacement. Microphones and hydrophones have problems distinguishing between cavitation noise and background noise, often creating a false or ambiguous signal. Hence, these technologies are not suitable for long-term, continuous cavitation monitoring of plants. Furthermore, since many of these detectors are not portable or retrofitable, their routine maintenance requires costly facility shutdowns.